Genetic Testing

My family has never had any major genetic issues with babies. My sister had a baby five years ago and when it was 6 months old, they discovered he has menkes disease. It’s a terminal disease and he was expected to live to 4 and has no quality of life. He doesn’t know who we are, he can’t talk, he can’t feed himself, he can’t move, he has seizures all the time and today he got a trach put in because he’s been in the hospital for a month & a half. It took them from the time he was six months old to about three months ago to find the DNA strand that made this happen. My sister believes it was just a freak thing that happened. It carries in girls and is active in boys only. Anytime I mention the information I need to get tested she acts stingy about it and just tells me it was a freak thing and I’m sure we don’t have it. 🤷🏽‍♀️ Which may be true. We’ve never had this before and my grandmother had a boy, my aunt had 3 boys, all of which have been fine. My question is... do I just let this be? My husband doesn’t seem concerned with it and we’re currently trying.